One known process for the preparation of polyimides is solution polymerization. This process includes a first step, in which a diamine compound and a dianhydride compound react in solution to form a partially polymerized polyimide, and a second step, in which the polymerization is completed and solvent is removed. The second step typically requires a residence time on the order of hours, and it also requires substantial capital investment in the form of gear pumps required to transport the molten polymer, and evaporation equipment required to remove the solvent and any water formed as a product of the polymerization and imidization reactions. This process allows carefully controlled proportions of the diamine and dianhydride reactant, but its long residence times, high temperatures, and exposure of the molten polyimide to atmospheric air often result in the degradation of the polyimide and its physical properties.
Another known process for the preparation of polyimides is melt polymerization. Melt polymerization is conducted in a single reactive extrusion step in which the diamine and dianhydride reactants are fed to an extruder, where they react to form a polyimide. This process is substantially faster and less capital-intensive than solution polymerization. It is also useful for the preparation of high molecular weight polyimides, as well as polyimides having high glass transition temperatures. However, it does not allow for precise control of reactant stoichiometry. As a result the polyimide produced is not consistent in quality. A further disadvantage of conventional melt polymerization techniques is that the reaction mixture passes through a so-called “cement stage” as the polyamic acid intermediate is formed. During this phase of the reaction, the mixture of reactants becomes very viscous and thus difficult to process. Because of these disadvantages, melt polymerization is not widely practiced commercially.
It would therefore be advantageous to synthesize polyimide by a method that provides the precise control of reactant stoichiometry and manageable viscosities offered by the solution polymerization, as well as the substantial time and equipment cost savings offered by melt polymerization.